Fluidized bed without a distributor

ABSTRACT

A fluidized chamber with a coarse layer of refractory particles and a fine layer of refractory particles along with a fluidizing gas inlet connected to a fluidizing gas supply. The fluidizing bed does not have a distributor and fluidizing gas is passed through the fluidized bed at a flow rate to fluidize the fine layer particles but not to fluidize the coarse layer. Preferably, the fluidized bed is a heat treating furnace, the fluidizing gas is combustible and objects to be heat treated are fed through the fluidizing chamber on a continuous basis. A method of operating a fluidized bed is also provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a fluidized bed furnace with a fluidizing chamber containing a coarse layer of unfluidizable material located beneath a fine layer of fluidizable material and no distributor to receive a premixed air/gas mixture. Preferably, the fluidized bed is a heat treating furnace.

2. Description of the Prior Art

Fluidized beds have been used for various purposes for many years for many uses including chemical processing and treating waste. Fluidized beds have been used for heat treating wires using a mixture of air and natural gas which is ignited in the bed to provide heat. Previous designs use two types distributor. One type of distributor is a porous tile located on top of an air plenum to allow air to enter the bed and a perforated pipe sitting above the distributor feeds natural gas premixed with some air into the fluidized bed, the total air being sufficient to burn the natural gas. The perforated pipe is usually buried in a layer of coarse sand to protect the perforated pipe and the porous tile from heat generated in the hot fluidized sand above the coarse layer. It is also known to use perforated pipes or perforated metal plates as the distributor.

The porous ceramic tiles are cemented in a steel frame work using a high temperature cement, which is fairly brittle. If the tiles and the steel frame work get too hot, the cement will crack and cooling air must be circulated through the support steel. In addition, an emergency blower that will keep air flowing in case of power failure must be installed. Both of these requirements are expensive. When the natural gas is introduced above the bed through perforated pipes, many of the advantages of the porous tile distributor are lost. Lateral mixing of air and gas in a fluidized bed is fairly poor, so this design does suffer from non-uniform combustions resulting in high emissions and low combustion efficiency. The porous tiles are expensive as is their installation. When perforated pipes are used as the distributor (with no porous tiles), a large number of perforated pipes are required and the pipes are made of stainless steel and are very costly and labour-intensive to produce. Small holes are drilled in the tubing at approximately one inch spacing.

Drawn steel wires can also be heat treated using a bath of molten lead to heat the wires to the annealing or stress-relieving temperature of around six hundred to one thousand degrees Celsius (600-1000° C.). The wires are submerged in the molten lead using a cast ceramic sinker while the wires are pulled through it at a fixed speed that varies with wire diameter. While this system is simple and effective, many jurisdictions have restricted or banned the use of lead due to its toxicity and environmental concerns.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a fluidized bed that does not have a distributor and passes a fluidizing gas directly through a coarse layer of unfluidizable material located beneath a fine layer of fluidizable material.

It is an object of the present invention to provide a fluidized bed heat treating furnace that does not have a distributor and passes an air/gas mixture directly through a coarse layer of unfluidizable material located beneath a fine layer of fluidizable material.

A fluidized bed comprising:

-   -   (a) a fluidizing chamber having a fluidizing gas entrance         therein;     -   (b) a coarse layer of refractory particles of sufficiently large         size so as to remain unfluidized when the bed is operating, the         coarse layer being located in the chamber above the entrance;     -   (c) a fine layer of refractory particles of sufficiently small         size so as to be fluidized when the bed is operating, the fine         layer being located immediately above the coarse layer when the         bed is not operating;     -   (d) a fluidized zone located above the coarse layer when the bed         is operating and formed by particles of the fine layer;     -   (e) the fluidizing gas entrance being connected to a supply of         fluidizing gas mixture;     -   (f) a blower to cause the fluidizing gas to flow through the         entrance and through the coarse layer and the fine layer without         passing through a distributor at a flow rate to fluidize the         fine layer but not the coarse layer; and     -   (g) an access to an interior of the chamber.

Preferably, the fluidized bed is a fluidized bed heat treating furnace and the fluidizing gas is a premixed air/gas mixture, the mixture being combustible, the fluidized bed having an igniter to ignite the air/gas mixture.

A method of operating a fluidized bed, said bed having a fluidizing chamber with a fluidizing gas inlet and a bottom thereof, the method comprising:

-   -   (a) locating a layer of coarse refractory particles of         sufficient size so as to remain unfluidized when said bed is         operating, the coarse layer being located in a bottom of the         chamber above the inlet;     -   (b) locating a fine layer of refractory particles of a size so         as to be fluidized above the coarse layer when said bed is         operating, said fine layer being located immediately above said         coarse layer when said bed is not operating;     -   (c) connecting the fluidizing gas inlet to a supply of         fluidizing gas;     -   (d) connecting a blower to pass the fluidizing gas through the         fluidizing gas inlet and through the coarse layer and the fine         layer without passing through a distributor at a velocity to         fluidize the fine layer but not to fluidize the coarse layer;     -   (e) choosing the particles of the coarse layer so that a spacing         between the particles is sufficiently small that combustion does         not occur in the coarse layer; and     -   (f) operating the fluidized bed at a temperature, flow rate and         duration to achieve a desired result.

Preferably, the method of operating a fluidized bed comprises operating a fluidized bed heat treating furnace to heat objects and connecting the premixed air/gas inlet to a supply of combustible gas into a source of air, premixing the air and gas to form a premixed air/gas mixture that is combustible, locating objects to be heat treated within the chamber to be within the fluidizing zone when the bed is operating. The method comprises igniting the air/gas mixture as the mixture exits from the coarse layer and operating the fluidized bed at a temperature, flow rating duration to heat treat the objects.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial sectional side view of a fluidized bed heat treating furnace; and

FIG. 2 is a schematic sectional front view of the fluidized bed heat treating furnace with additional features to that shown in FIG. 1.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

In FIG. 1, a heat treating furnace 2 that is a fluidized bed has a fluidizing chamber 4 with a premixed air/gas entrance 6 located in a base 8 of the fluidizing chamber 4. The chamber 4 has side walls 10 and a removable top 12 that can be opened to provide access to the chamber 4. The side walls 10 and top 12 are preferably insulated by insulation 13. In a bottom of the chamber 4, above the premixed air/gas entrance 6 and supported by the base 8 is a coarse layer 14 of refractory particles of sufficiently large size so as to remain unfluidized when the bed 2 is operating.

A fine layer 16 of refractory particles of sufficiently small size so as to be fluidized when the bed 2 is operating is located immediately above the coarse layer 14. A plurality of wires 18 to be heat treated are mounted within the fluidizing chamber 4 at a height that is above the fine layer 16 when the bed is not operating, but is within a fluidized zone and is contacted by the particles of the fine layer 16 when the bed is fluidized. An interior surface of the side walls 10 is coated with a heat resistant material 20. The top 12 has vents to exhaust gases from a fuel/air mixture (not shown in FIG. 1) that is introduced under pressure through the air/gas entrance 6. A refractory particle back flow preventer 22 is located just inside the entrance 6. Steel supports 24 support the base and smaller steel supports 26 provide longitudinal rigidity for the side walls 10.

In FIG. 2, the fluidized bed heat treating furnace 2 has a plurality of premixed air/gas entrances 6 connected to a manifold 30, which is in turn connected to a blower 32 to pressurize the premixed air/gas mixture (not shown) and force the mixture into the fluidizing chamber 4 at a sufficient velocity to fluidize the fine layer 16, but leave the coarse layer 14 unfluidized. The top 12 has a plurality of vents 34 therein to exhaust the spent fuel/gas mixture (not shown). The vents 34 are not shown in FIG. 1. The wires 18 (only one of which is shown in FIG. 2) extend outside the chamber 4 and can be fed continuously through the chamber 4 while the bed is operating to heat treat the wires. The same reference numerals are used in FIG. 2 for those components that are identical in FIG. 1.

Depending on the size of the fluidized bed heat treating furnace, the number of premixed air/gas inlets can range from one to several inlets to insure that the premixed air/gas mixture is evenly distributed when it exits the coarse layer 14. The furnace 2 does not have a distributor and therefore has several advantages over previous fluidized bed heat treating furnaces. In the furnace of the present invention, no cooling air is required and there is no distributor or perforated pipes or tubes. Since no cooling air is required and the pressure drop across the two layers 14, 16 is much lower than previous furnaces, a smaller blower size can be used for the same furnace area of the present invention than for previous furnaces. This results in lower energy consumption, a higher combustion efficiency and fewer contaminants being exhausted through the vents 34 as more complete burning is achieved by the furnace of the present invention than is achieved by previous furnaces. Also, the explosion hazards that can be present with previous furnaces are eliminated by the furnace of the present invention. Further, the furnace of the present invention is much less expensive to construct and operate than previous furnaces. Maintenance costs of the furnaces of the present invention is much less than the maintenance costs of previous fluidized bed heat treating furnaces because no distributor is present.

The size of the particles in the coarse layer is chosen to be less than the quenching distance of the fuel. When the fuel is natural gas and air, the quenching distance is three millimetres (3 mm). The spacing between the particles of the coarse layer is therefore less than three millimetres (3 mm) so that no combustion will occur within the coarse layer. Combustion occurs in the fine layer, which is fluidized. Preferably, the coarse layer and fine layer are particles of sand. The coarse layer has sand particles that are sufficiently large not to be fluidized at the flow rates used to fluidized the finer sand located above the coarse layer. The air/gas mixture entering the coarse sand distributes uniformly in the lateral direction due to the low flow resistance of the coarse grit sand coupled with the increasing flow area available as radial distance from the inlet increases. By the time the air/gas mixture reaches the fine fluidizing sand, it is uniformly distributed and at an even pressure across the entire coarse grit sand layer.

Preferably, the coarse grit sand of the coarse layer is the same material as the fine fluidizing sand of the fine layer so that the coarse layer can withstand the same temperatures as the fine layer and does not need to be protected by heat in any way. While the coarse layer and fine layer preferably consist of the same material, the particles of the coarse layer, of necessity, are larger than the particles of the fine layer. It is not essential that the coarse grit sand be the same material as the fine fluidizing sand. For example, the coarse grit layer can be gravel. Experiments have shown that even with the fine fluidized sand at a thousand degrees Celsius (1000° C.), the coarse grit sand one inch below the coarse/fine sand interface is below one hundred degrees Celsius (100° C.) and cannot support combustion.

The heat treating furnace of the present invention can accommodate a number of wires to be heat treated (for example twenty to fifty) running in parallel approximately twenty-five to forty millimetres (25-40 mm) apart. The fluidized bed of the present invention can be operated between six hundred degrees Celsius (600° C.) and one thousand degrees Celsius (1000° C.) and range in length from around fifteen (15) feet long to over eighty (80) feet long depending on wire speed. The bed is typically around three (3) feet wide but the width varies with the number of wires and the space in between the wires. The wires are preferably drawn through the fluidizing zone and extend through suitable openings (not shown) in end walls 36 of the chamber 4 on a continuous basis using pulleys located outside the chamber. The wires are pulled through the fluidizing chamber at a height to be within the fluidized portion of the fine layer particles when the bed is operating. Preferably, the heat resistant material 20 is a steel alloy. 

1. A fluidized bed comprising: (a) a fluidizing chamber having a fluidizing inlet therein; (b) a coarse layer of refractory particles of sufficiently large size so as to remain unfluidized when said bed is operating, the coarse layer being located in the chamber above the inlet; (c) a fine layer of refractory particles of sufficiently small size so as to be fluidized when said bed is operating, said fine layer being located in the chamber immediately above said coarse layer; (d) said particles of said fine layer being located in a fluidized zone above said coarse layer when said bed is operating; (e) the gas inlet being connected to a supply of fluidizing gas; (f) a blower to cause the fluidizing gas to flow through the fluidizing gas inlet and through the coarse layer and fine layer without passing through a distributor at a flow rate to fluidize the fine layer but not the coarse layer; and (g) an access to an interior of the chamber.
 2. A fluidized bed as claimed in claim 1 wherein the fluidizing gas is a premixed air/gas mixture.
 3. A fluidized bed as claimed in claim 2 wherein the premixed air/gas mixture is combustible and the fluidized bed is a heat treating furnace.
 4. A fluidized bed as claimed in claim 3 wherein the bed has an igniter to ignite the air/gas mixture as the mixture exits the coarse layer, a spacing between the particles of the coarse layer being sufficiently small to prevent combustion from occurring within the coarse layer.
 5. A fluidized bed as claimed in claim 4 wherein a size of the particles in the coarse layer is chosen to be less than the quenching distance of the air/gas mixture.
 6. A fluidized bed as claimed in claim 5 wherein the fine layer is comprised of fine particles of sand and the coarse layer is comprised of coarse particles of sand.
 7. A fluidized bed as claimed in claim 6 wherein the coarse layer is coarse grit sand.
 8. A fluidized bed as claimed in claim 7 wherein the material of the coarse layer is the same material as the material of the fine layer with a different particle size.
 9. A fluidized bed as claimed in claim 4 wherein the access is several aligned openings in walls of the fluidizing chamber with each pair of openings on opposing walls being sized to receive wires to be heat treated.
 10. A fluidized bed heat treating furnace comprising: (a) a fluidizing chamber having a premixed air/gas inlet therein; (b) a coarse layer of refractory particles of sufficiently large size so as to remain unfluidized when said bed is operating, the coarse layer being located in the chamber above the inlet; (c) a fine layer of refractory particles of sufficiently small size so as to be fluidized when said bed is operating, said fine layer being located in the chamber immediately above said coarse layer; (d) said particles of said fine layer being located in a fluidized zone above said coarse layer when said bed is operating; (e) the premixed air/gas inlet being connected to a supply of premixed air/gas mixture, the mixture being combustible; (f) a blower to cause the air/gas mixture to flow through the premixed air/gas inlet and through the coarse layer and fine layer at a flow rate to fluidize the fine layer but not the coarse layer; (g) an igniter to ignite the air/gas mixture as the mixture exits the coarse layer, a spacing between the particles of the coarse layer being sufficiently small to prevent combustion from occurring within the coarse layer; (h) at least one retainer to support objects to be heat treated within the fluidized zone of the chamber; and (i) the chamber having an access to an interior thereof.
 11. A fluidized bed as claimed in claim 9 wherein the furnace is constructed so that the premixed air/gas mixture does not pass through a distributor.
 12. A method of operating a fluidized bed, said bed having a fluidizing chamber with a fluidizing gas inlet and a bottom thereof, the method comprising: (a) locating a layer of coarse refractory particles of sufficient size so as to remain unfluidized when said bed is operating, the coarse layer being located in a bottom of the chamber above the inlet; (b) locating a fine layer of refractory particles of a size so as to be fluidized above the coarse layer when said bed is operating, said fine layer being located immediately above said coarse layer when said bed is not operating; (c) connecting the fluidizing gas inlet to a supply of fluidizing gas; (d) connecting a blower to pass the fluidizing gas through the fluidizing gas inlet and through the coarse layer and the fine layer without passing through a distributor at a velocity to fluidize the fine layer but not to fluidize the coarse layer; (e) choosing the particles of the coarse layer so that a spacing between the particles is sufficiently small that combustion does not occur in the coarse layer; and (f) operating the fluidized bed at a temperature, flow rate and duration to achieve a desired result.
 13. A method of operating a fluidized bed as claimed in claim 12 including the step of choosing a size of the particles in the coarse layer to be less than the quenching distance of the air/gas mixture.
 14. A method of operating a fluidized bed as claimed in claim 12 including the step of choosing the fine layer to be comprised of fine particles of sand in choosing the coarse layer of sand to be comprised of coarse particles of sand.
 15. A method of operating a fluidized bed as claimed in claim 14 including the step of choosing the coarse layer to be coarse grit sand.
 16. A method of operating a fluidized bed as claimed in claim 12 including the step of choosing the material of the coarse layer to be the same material as the material of the fine layer with a different particle size between the coarse layers and the fine layer.
 17. A method of operating a fluidize bed as claimed in claim 12 including the step of having the access have several aligned openings in walls of the fluidizing chamber with each pair of openings on the opposing walls being sized to receive wires to be heat treated.
 18. A method of operating a fluidized bed heat treating furnace to heat objects, said furnace having a fluidizing chamber with a premixed air/gas inlet in a bottom thereof, said method comprising: (a) locating a layer of coarse refractory particles of sufficient size so as to remain unfluidized when said bed is operating, the coarse layer being located in a bottom of the chamber above the inlet; (b) locating a fine layer of refractory particles of a size so as to be fluidized above the coarse layer when said bed is operating, said fine layer being located immediately above said coarse layer when said bed is not operating; (c) connecting the premixed air/gas inlet to a supply of combustible gas and to a source of air, premixing the air and gas to form a premixed air/gas mixture that is combustible; (d) locating objects to be heat treated within said chamber to be within said fluidizing zone when said bed is operating; (e) connecting a blower to pass the air/gas mixture through the premixed air/gas inlet and through said coarse layer and said fine layer at a velocity to fluidize said fine layer, but not to fluidize said coarse layer; (f) choosing the particles of said coarse layer so that a spacing between the particles is sufficiently small so that the combustion does not occur in the coarse layer; (g) igniting the air/gas mixture as the mixture exits from said coarse layer; and (h) operating the fluidized bed at a temperature, flow-rate and duration to heat treat the objects.
 19. A method as claimed in claim 18 wherein said objects to be heat treated are passed through said access on a continuous basis. 